Platform operating system



June 1963 J. B. MCALPINE PLATFORM OPERATING SYSTEM 4 Sheets-Sheet 1 Filed 001- 21, 1960 54 46 48 44 LS-i June 18, 1963 v B MCALPINE PLATFORM OPERATING SYSTEM Filed Oct. 21. 1960 4 Sheets-Sheet 2 P MP June 1963 J. a. MGALPINE PLATFORM ommmc sysma 4 Sheets-Sheet 4 Filed 001;. 21, 1960 Inch.

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United States Patent 0,

3,094,192 PLATFORM OPERATING SYSTEM John B. McAlpine, Scarborough, Ontario, Canada, as-

signor to Symington Wayne Corporation, Salisbury, Md., a corporation of Maryland Filed Oct. 21, 1969, Ser. No. 64,076 16 Claims. (Cl. 137-228) This invention relates to a system for operating a vertically-movable platform, and it more particularly relates to such a system for lowering the undercarriage of an air craft within a pit.

Modern aircraft, such as jet transports, have become quite large and heavy; and it has, therefore, become quite difficult to repair and test their undercarriage through its operating range. Proposals have, therefore, been made to drop the wheels within pits while an aircraft is supported in a raised position upon jacks. In dropping the undercarriage, the wheels are positioned upon platforms raised substantially to ground level over the pits, and then the platforms are raised a short distance to permit supponting jacks to be Placed under the airplanes jacl irlg points. Then the platforms are lowered far enough mto the pit to allow full extension, maintenance, testing and replacement of the wheel assemblies if necessary.

Suitable platforms for such operation should be relatively large in area for accommodating the large and rather spread out wheel assemblies of big airplanes, and they must move upwardly and downwardly through a stroke of considerable length. This imposes severe loads upon suitable raising and lowering devices such as bydraulic piston and cylinder assemblies, partlcularly when the load is eccentrieally applied to the platforms in rolling the wheels on and off them.

These severe roll-on and roll-off loads can be resisted by supporting such platforms upon several piston and cylinder assemblies as described in US. Letters Patent 2,803,360. However, this complicates the manufacture, operation and maintenance of the lifting system as well as making it necessary to synchronize operation of the individual lifting assemblies. Furthermore, even when several lifting elements are utilized, they might all fail simultaneously, or failure of one element might disasterously tilt the platform or damage the operative lifting element.

An object of this invention is to provide a safe, simple and economical arrangement for operating a verticallymovable platform.

Another object is to provide such an arrangement for installation within 'a pit which accommodates the undercarriage of an aircraft.

In accordance with this invention a platform is mounted upon a piston and cylinder assembly installed, for example, within a pit. An actuating device extends and retracts locking bars laterally from the platform for engagement upon a supporting surface disposed at the side of the pit adiacent ground or loading level. I lydra-ul1c pressure bleeding means is connected with the piston and cylinder assembly for gently lowering the platform onto the supporting surface, and this pressure bleeding means 1s actuated by the platform control system to lower the extended locking bars onto the supporting surface when the platform is substantially at ground level. The support ng surface may also include vertical surfaces between WhlCh the locking bars engage to prevent any hor zontal as well as vertical movement of the platform. Plus msures that any roll-on and roll-off forces imposed upon the platform are resisted by the supporting surfaces and not 11111306611 upon the piston and cylinder assembly.

The pressure bleeding means maybe inactivated when the platform is raised a predetermlned d1stance above the loading level to permit it to lift and maintain a load Patented June 18, 1963 above this level. Furfltennore, interlocking means may be provided in the control system for preventing movement of the platform a substantial distance above the loading level unless the locking bars are extended and for preventing movement of the platform below the loading level unless the locking bars are retracted. The locking bar assembly may be hydraulically actuated from the main source of hydraulic pressure, and the hydraulic system may utilize solenoid-operated valves for controlling the raising, lowering and bleeding of the piston and cylinder assembly.

Novel features and advantages of the present invention will become apparent to one skilled in the art from a reading of the following description in conjunction with the accompanying drawings wherein similar reference characters refer to simlar parts and in which:

FIG. 1 is a cross-sectional view in elevation of one embodiment of this invention in the lowered condition;

FIG. 2 is a cross-sectional view in elevation of the upper portion of the embodiment shown in FIG. 1 with the platform substantially at ground level shown in full outline and above ground level partially illustrated in phantom outline;

FIG. 3 is a plan view of the embodiment shown in FIG. 2;

FIG. 4 is a schematic diagram of a hydnaulic system for operating the embodiment shown in FIG. 1;

FIG. 5 is a schematic diagram of the embodiment shown in FIG. 1 together with portions of its control system;

FIG. 6 is a schematic electrical diagram for operating the embodiment shown in FIGS. 1-4, including its hydraulic and control system;

FIG. 7 is a schematic chart of the operational phases of portions of the electrical and mechanical control system described in FIG. 5 with particular reference to sensing devices;

FIG. 8 is an enlarged view in elevation of a portion of the control system shown in FIG. 1;

1:16. 9 is a plan view of the portion shown in FIG. 8; an

FIG. 10 is a cross-sectional diagrammatic view taken through FIG. 9 along the line 101[l.

In FIGS. 1 and 2 is shown a vertically-movable platform arrangement 10 including a hydraulic piston and cylinder assembly 12 mounted upon the base or founda- .tion 14 of a pit .16 which is, for example, of rectangular plan, and extends downwardly from the floor or ground level 18. Piston and cylinder assembly 12 is of the hydraulic ram type, and it is, for example, rigidly and economically madein the manner described in copending application for US. Letters Patent Serial No. 57,325, filed September 20, 1960. Cylinder and piston assembly 12 is secured to foundation 14 by anchor bolts 20 through supporting beams 22, and the lower end 24 of assembly 12 rests detachably upon a base plate 26 which is anchored within a concrete floor 28 which fills the bottom of casing 30 accommodating piston and cylinder assembly 12. Details of the aforementioned portions of piston and cylinder assembly 12, its mode of construction and installation are fully described in the aforementioned oopending application for US. Letters Patent.

A platform 32 is mounted upon piston 34 which has a stroke sufficient to raise platform 32 from the lower position shown in FIG. 1 resting upon stops 36 to a position above ground level 18 shown in phantom outline in FIG. 2 in which, for example, the upper surface of platform 32 is nine inches above ground level 18. A locking bar assembly 38 is connected to platform 32 by installation within its structural frame. Locking bar assembly 38 includes locking bars 40 which are mounted to laterally reciprocate through the sides of platform 32 upon sliding bearings 42 which are, for example, antifriction bearings of the ball bearing type. An actuating device 44 is connected to locking bars 40, and it includes an actuating cylinder 46 from which a rod 48 extends. Rod 48 is rotatably connected to bell crank 50 which is pivoted upon a shaft within platform 32. A pair of connecting rods 52 connect crank 50 with locking bars to extend and retract them in response to the rotational movement of hell crank imparted by movement of rod 48 by actuating cylinder 46. The end 54 of actuating cylinder 46 remote from rod 48 is also rotatably mounted to permit actuating device 44 to move through its operative position. A pair of actuating devices 44 are provided in conjunction with the two locking bars 40 that each operate to provide the four locking bars 40' in all associated with platform 32 as shown in FIG. 3. However, it is also possible to utilize a single actuating device 44 for operating both pairs of locking bars 40.

In FIG. 1 locking bars 40 are shown in the retracted condition to permit platform 32 to be lowered within pit 16. In FIG. 2 locking bars 40 are extended and resting upon horizontal supporting surfaces 56 to maintain the upper surface 58 of platform 32 substantially at ground level 18. Platform 32 is thereby supported by horizontal surfaces 56 to prevent any eccentric loads transmitted to the platform at ground level, for example, by the wheels of the aircraft rolling across the platform from being transmitted to cylinder and piston assembly 12. Furthermore, the vertical ends of bars 40 in FlG. 2 are extended into contact with vertical stop surfaces 57 to prevent platform 32 from shifting horizontally as a load is moved across it. The straight line disposition of bars 40, the connected links of bell crank 50 and connecting rods 52 in this extended condition rigidly align these elements to resist any horizontal shifting forces.

A position of the platform slightly above ground level which is designated by dashed line 60 in FIG. 2 is a control position at which locking and unlocking of the platform occurs, and the various events which occur at this position are later described in detail in conjunction with the electrical and mechanical control system. Platform 32 can also be raised to an extreme upper position designated in phantom outline in FIG. 2. As platform 32 rises to this highest position which is, for example, nine inches or more above ground level, extended locking bars 40 lift pivoted plates 62 covering recesses 64 upwardly. Plates 62 cover recesses 64 when platform 32 is below ground level.

Also shown in FIG. 2 and schematically in FIG. 5 are a pair of electrical limit switches LS-3 and LS-4 which are actuated by a projection 66 mounted on one of locking bars 40 to detect whether locking bars 40 are in the retracted or extended condition. In FIG. 1 together with FIGS. 8-10 are shown another pair of limit switches LS-2 and LS-1 associated with a telescopic tripping rod assembly 68. Limit switches LS-l and LS-2 actuate the control circuit in accordance with the vertical position of platform 32 in a manner later described in detail. Telescopic assembly 68 as shown in FIGS. 8-10, includes a rod 70 connected to a lower portion of platform 32. Rod 70 slides within a sleeve 72 and includes a projection 74 at its lower end which engages the upper end 76 of sleeve 72 when rod 70 is lifted sufliciently. The lower end 78 of sleeve 72 is resiliently urged downwardly by coil spring 80 which reacts between shoulder 82 upon enlarged end 78 and the upper end 84 of pedestal 85 which is secured to supporting beams 22. Limit switches LS-1 and LS-2 are respectively actuated by surfaces 86 and 88 of sleeve 72 in the sequence and manner later described in detail in conjunction with the control system.

In FIG. 4 is shown a schematic diagram of the hy draulic system 90 for operating piston and cylinder as sembly 12 and locking bar actuating cylinder 46. Hydraulic system 90 includes a pump 92 driven by electric motor 94 with its suction connected to oil reservoir 96 through a strainer 98. Pump 92 discharges through pressure piping 100, which is also connected to cylinder and piston assembly 12, through a paralleled pair of solenoid-operated valves S-1 and 8-2 and check valves 102 and 104 in front of them. Solenoid-operated valve S-l has a greater flow capacity than valve S-2. Valve S-1 is used as the main piston raising valve, and S2 as an auxiliary or inching-up valve. A manually adjustable valve 106 is connected in front of valve 5-2 to restrict flow through 5-2 by bypassing through relief valve 116.

Lowering of the piston and platform is accomplished through another pair of paralleled solenoid-operated valves S3 and 8-4, which are connected to piping 108 directly connected to cylinder and piston assembly 12. Valve 8-3 is the main valve and valve 5-4 is a smaller capacity auxiliary valve for controlling the return flow of oil through pipe 110 to oil reservoir '96. A manually operated valve 112 is connected in front of valve 8-4 to permit manual determination of the lowering speed of the platform for inching down. Valve 114 at the top of cylinder and piston assembly 12 permits air to be bled from the highest point of the system during installation. A relief valve 116 is also connected between discharge line 100 and reservoir 96 to recirculate the discharge from pump 92 when the pressure in discharge line 100 is above a predetermined maximum and during the inching up operation.

Auxiliary pipe line 118 connects locking bar actuating cylinder 46 to pressure line 100 through solenoid-operated valve 8-5 which is a four-way solenoid-operated valve including sections S-Sa and b. Pressure line 118 is connected through section S-5b to the closed end 120 of piston 122 within cylinder 46 to move it in a direction to retract locking bars 40. The other side of piston 122 to which to rod 48 is attached is connected through return line 124 to valve section 5-5:: from which return line 126 is connected to reservoir 96. Valve section S-Sa accordingly controls the return of fluid from the rod side of cylinder 46, and valve section S5b controls the flow of oil to the blank side of cylinder 46. This causes the speed of retraction controlled by valve section S-5b to be greater than the speed of extension of locking bars 40 controlled by valve section S-5a because of the greater area of the blank side of piston 122 in comparison with the side to which rod 48 is attached.

FIG. 5 is a schematic diagram showing the relative positions of cylinder and piston assembly 12, platform 32 and portions of the control system including limit switches LS-1-4, actuating rod assembly 68 including surfaces 86 and 88, and actuating projection 66 upon one of locking bars 40. In FIG. 5 the position of platform 32 substantially in line with ground level 18 is shown in full outline together with the fully raised position of platform 32 shown in phantom outline. Reference is made to FIG. 5 in conjunction with the following description of FIG. 6.

In FIG. 6 is shown a schematic diagram of the electrical portions of the control circuit for raising and lowering platform 32 both at full and inching speeds and for extending and retracting locking bars 40. FIG. 6 also describes a provision for bleeding hydraulic pressure from cylinder and piston assembly 12 through lowering valve S3 or alternatively through smaller capacity lowering valve 8-4. This hydraulic pressure bleeding means is actuated when platform 32 is raised within a predetermined distance above ground level with locking bars 40 extended for insuring that the weight of the platform and any loads thereon are supported by stationary supporting surfaces 56 and not transmitted to the cylinder and piston assembly 12 by roll-on and roll-off loads when the platform is substantially at ground level. The bleeding means is energized when platform 32 is in condition for raising above ground level 18 and is, for example, actuated when locking bars 40 are extended. However, limit switch LS-2 provides a means for de-activating the bleeding means when platform 32 rises above a predetermined distance above ground level such as A inch to permit the platform and loads imposed upon it to be raised and maintained distances above ground level such as between A inch and 9 inches. Even though the bleeding means is operative up to inch positions of platform 32 above ground level, the flow of pressure oil to cylinder and piston assembly 12 is sufiiciently greater than the amount of oil being bled to raise platform 32 while a small amount of oil is continuously bleeding back to reservoir 96. The other portions of the control circuit will be described in conjunction with the various events that they control.

In FIG. 7 is shown a diagram of the phases of operation of the various limit switches and relays controlling the events controlled by the diagram shown in FIGS. 4-6.

In describing the circuit shown in FIG. 6, the sequence of operation of the system is described assuming that platform 32 is first disposed at its lowest position as shown in FIG. 1. The following outline describes the operation of each of the buttons and control elements in accordance with its mode of connection through all of the operational conditions of platform 32. FIG. 6 includes relays R1R6 which control various contactors designated by alphabetical sufiixes to perform the described operations.

OPERATION OF PLATFORM CONTROL SYSTEM (1) Raise Button The raise button 128 controls the vertical position of the platform over the full travel of the hydraulic plunger, however the platform will not rise above ground level until locking arms 40 have been extended. Contact must be sustained on this button, otherwise platform motion will stop.

Depressing raise button 128 initiates the following:

(a) Energizes R1 through N.C. (normally closed con tacts RSA (LS-1 unactuated) with platform below floor and ND. (normally open) contacts RGA (LS3 actuated) above floor;

(b) Starts motor through RIA;

(c) Opens solenoid S-l through RIB; and

(d) Prevents automatic bleeding by opening contacts RIC.

This feeds pressure fluid to the plunger, causing the platform to rise until LS-l is actuated at a point /8" above floor level.

(2) Limit Switch LS-I Limit switch LS-l is set to be tripped at a point Vs" above the floor level by surface 86, and this energizes relay R initiating:

(a) De-energizing R1 (and R2) through N.C. RSA;

(b) Unlocking circuit with R3 (to extend arms) through RSB;

(c) Breaking circuit with S1 on unlocking cycle through RSC;

(d) Energizing solenoid S-Sb on unlocking cycle through RSD.

Platform has now reached its maximum height of travel with the arms retracted.

(3) Lock Button Depressing lock button 130 momentarily causes locking arms 40 to extend to their limit and the platform 32 to settle on its stops 56 level with the hangar floor. Momentary pressure on lock button 130:

(a) Energizes relay R3 through contacts RSB (closed by LS-1 being actuated) R60 (normally closed with LS-3 unactuated);

(b) Locks in R3 through contact R3A;

(c) Starts motor through contacts R3B; and

(d) Energizes S-Sa through contacts R3C.

This causes pressure fluid to extend locking arms 40. As locking arms 40 leave their retracted positions, LS4 relaxes, breaking the continuity of the lowering circuit through LS-4a and making the contact LS-4b in series with the unlocking circuit. Locking arms extend to their limit where switch LS-3 is actuated, causing:

(a) Relay R6 to energize;

(b) Contacts R6A to close for further raising of platform 32;

(c) Contacts R613 to close for lowering platform 32;

(d) Contacts RGC to open, thereby de-energizing R3; and

(e) Contacts R6D to close in series with bleed circuit.

Contacts R6C de-energize R3 stopping motor and closing solenoid valve 8-5, and since the bleed circuit is operative through R6D, the platform slowly settles on stops 56 coming to rest.

Since the platform may remain in this position for prolonged periods, a pressure switch P.S.1 is placed in series with R6 to de-energize the same when the system pressure drops below 15 lbs/sq. inch. 1.8-3, however, remains depressed, and any increase in system pressure such as caused by thermal expansion, will close contacts PS1 to open the bleed circuit as before.

Up to this point limit switch LS-2 has not been actuated, and its normally closed contacts have been in series with the unlocking circuit and the bleed circuit. It is not used in conjunction with a relay and provides the simple function of preventing automatic bleeding and unlocking the arms after the platform is raised approximately /4" above the floor.

(4) Raising and Lowering From /4 Inch to an Upper Level Platform 32 is level with hangar floor with arms extended and resting on stops 56. The system is bled and entire electrical circuit de-energize-d through pressure switch P.S.1 being released. Platform 32 may now be raised or lowered between this position and its upper limit. Pressure on the raise button will:

(a) Energize RI through contactor RSA; (b) Energize S1 through contactor RIB.

As soon as pressure is generated, the pressure switches contacts will close making circuit for Relay R1 through contacts REA. At Ms" above floor level LS-l is actuated to open contacts RSA; but since continuity has already been established through R6A, platform 32 will continue to rise. At this point relay R5 will pull in.

At A" above floor level limit switch LS-2 will be actuated isolating the bleed circuit and arms unlock circuit. The platform will rise to its upper limit against the internal mechanical stop. Continued pressure on raise button 128 will cause the pump to discharge its full capacity through relief valve 116 without damage to the system.

(5) Inching Up The inch up circuit is paralleled with the Raise circuit, and it is governed by the same interlocks and limit switches. Inching up is achieved by creating suflicient back pressure through solenoid valve S-2 and its associated needle valve to bypass a variable proportion of the pump delivery to the tank.

Constant pressure on inch up button energizes R2, and substituting 5-2 for 8-1, RZA, R213 and RZC for RlA, RIB and RIC, the same conditions prevail as previously described. At any point between floor level and 4" above floor level, the automatic bleed system will be operative, and the platform will slowly settle to rest on its stops. At any point above A1", the platform will hold its position. The platform can now be raised to its upper limit of travel against internal mechanical stop.

(6) Lowering Platform A constant pressure on lowering button 134 will:

(a) Energize solenoid valve S-3 through contacts R6B;

(b) Stop at any point when button is released; and

() Automatic bleed circuit will become operative at A1" above the floor level causing the platform coming to rest on rocking arms.

(7) Unlocking Platform A momentary pressure on unlock button 136 will cause the platform to rise A? and retract locking arms 40, provided LS-Z is released. Pressing unlock button 136 will:

This causes platform 32 to rise until LS-1 is actuated, when:

(a) Solenoid S1 is de-energized stopping platform 32 by opening contactor RSC; (b) Solenoid S-Sb is energized by closing contacts RD.

Arms start to retract, relaxing limit switch LS-3 causing:

(1) Relay R6 to trip;

(2) Opening of contact R6A to prevent raising platform;

(3) Opening contact R613 to prevent lowering platform;

(4) Closing contacts R6C in lock circuit; and

(5) Opening contacts R6D in bleed circuit.

When arms 40 are fully retracted, limit switch LS-4 is actuated making a circuit with 5-3 and 8-4 to allow further lowering of platform and tripping relay R4.

(8) Lowering Platform Below Floor Level A constant pressure on lower button 134- energizes S-3 through limit switch LS-4, causing the platform to descend to its lower limit where it comes to rest on external mechanical stops.

(9) Inch Down The inch down button 138 is paralleled with Low-er button 134. Constant pressure on inch down button 138 produces a slower descending speed through its smaller solenoid valve S-4 and associated adjustable throttle valve 112.

(10) Bleed Circuit The bleed circuit is incorporated to insure that when the platform is level with the floor and locked, any rollover loads will be supported on the locking arms and not on the hydraulic plunger. This circuit is operative between floor level and /4" above floor level, and for continuity requires:

(a) Limit switch LS-Z relaxed;

(b) Relay R4 relaxed;

(0) Relay R6 relaxed;

(d) Relay R2 relaxed;

(e) Relay R1 relaxed; and

(f) Pressure switch P.S.1 actuated (above psi.)

OPERATION OF HYDRAULIC SYSTEM The mode of operation of hydraulic system 90 described in FIG. 4 in conjunction with the connection shown in FIGS. 5 and 6 is described in the following under the various headings which are related to the various phases of operation of the hydraulic system.

(1) Raising the Platform Depressing raise button 128: (a) Starts the electric motor; and (b) Opens solenoid valve S-1.

8 (2) Oil Flow (a) Rotation of the pump will cause a negative pressure or partial vacuum at the intake side, allowing the atmospheric pressure in reservoir 96 to force oil through strainer 98 and up to the pump gears where it is forced by meshing gear teeth around the outside periphery to the pump discharge. Since the pump is of the positive displacement type, the maximum pressure at the outlet will equal the minimum total resistance at any instant, and since relief valve 116 has a setting 25% higher than the maximum resistance to overcome, it should not bypass normally.

(b) Solenoid S1 is open making a passage for the oil through the two check valves and strainer to the hydraulic cylinder.

(0) Oil entering the hydraulic cylinder will cause the plunger to ascend by displacement. The maximum lifting capacity will be the product of the relief valve setting and the effective area of plunger. A small pressure drop will be experienced between the pump outlet and cylinder due to friction, however for practical purposes, this may be neglected.

(3) Inching Up the Platform ((1) Flow conditions are similar to those when raising the platform, only passage is through the manual throttle valve 106 and solenoid valve S4.

(11) Adjustment of throttle valve 106 varies the proportioning of the flow through S2 and the relief valve. Should the throttle valve be fully closed, the total discharge of the pump would bypass through the relief valve and no motion of the plunger would take place.

(4) Support of the Platform at Intermediate Positions (0) The back pressure of the oil due to weight of moving mass is contained by check valves and normally closed solenoid valves.

(b) Electrical failure at any time would cause all valves to close, containing the hydraulic medium in the cylinder assembly and pipe line, thus causing the system to fail safe.

(5) Lowering the Platform (a) The lower button controls solenoid 5-3 which when opened allows a passage for the oil contained in the cylinder back to the reservoir.

(b) This is a form of controlled gravity lowering, and the speed of lowering is a function of the total line and component losses.

(6) Inching Down (a) The inch down button controls small solenoid 3-4 which increases the total losses mentioned above.

(b) Further speed control is obtained by adjusting manual throttle valve 112.

(7) Locking the Platform (a) The platform is raised to a point A" above the floor (the point at which limit switch LS1 actuates and arrests the upward movement of the platform).

(b) The lock button is depressed momentarily, starting the motor and energizing S-Sa.

(0) Pressure fluid finds a passage from the pump through solenoid S5b to the rod side of the double acting piston 122 extending arms 40. Fluid from the blank side of the cylinder is returned to the reservoir 96 through a flow control valve and solenoid S-Sa.

(d) When the arms are fully extended, limit switch LS-4 is actuated opening solenoid valve S-4. Oil from the cylinder is bled to the reservoir 96 through S-4 relaxing the system pressure.

(8) Unlocking the Platform (a) Momentary contact on the unlock button automatically raises the platform /s" and retracts the locking arms.

9 (b) The following sequence takes place:

(1) The motor starts up, and solenoid 8-1 is energized lifting the platform /is;

(2) Limit switch LS-l is actuated de-energizing solenoid S-1 and energizing solenoid 8-5, the motor still running;

(3) Pressure fluid finds a passageway through solenoid 8-54 to the blank side of the double-acting piston 122 retracting the arms;

(4) Fluid in the rear side of the cylinder is returned to the reservoir 96 through a flow control valve and solenoid S-Sb.

(c) Speed of retraction is higher than extension due to the differential effect of the piston rod.

(9) Bleed Plug A bleed plug 114 is provided to release entrapped air during initial charging, however in operation, the cylinder is self-purging and should not require attention.

(10) Emergency Lowering 0] Platform (a) Manual lowering valves are not fitted to the power unit to eliminate the possibility of the interlocks being defeated.

(b) Solenoid valve S-1 is provided with a manual lowering adjustment. Turning this anti-clockwise allows the main valve to open, lowering the platform. This adjustment should only be used in cases of emergency. What is claimed is:

1. A vertically-movable platform arrangement comprising a hydraulic piston and cylinder assembly, a platform mounted 'upon said piston, said piston and cylin der assembly having a stroke sufiicient to raise said platform through a series of levels including a loading level, a locking bar assembly connected to said platform including locking bars which are extendable to project laterally from said platform, an actuating device connected to said locking bar assembly for extending said bars laterally from said platform and for retracting them, supporting surface means disposed at the side of said platform adjacent said loading level for engagement by said locking bars when they are extended, hydraulic pressure bleeding means connected to said piston and cylinder assembly for lowering said platform at a slow speed, control means for actuating said pressure bleeding means when said locking bars are extended above said supporting surface means and said platform is substantially aligned with said loading level to lower said platform upon said supporting surface means whereby any forces imposed by a load moving across said platform are resisted by said supporting surface means and said hydraulic piston and cylinder assembly is relieved of any loads and pressures, said control means being arranged for actuating said pressure-bleeding means when said platform is disposed within a predetermined distance above said loading level, and said control means being also arranged for inactivating said pressure bleeding means when said platform is more than said predetermined distance above said loading level to permit said platform to raise and maintain a load above said predetermined distance.

2. An arrangement as set forth in claim 1 wherein said control means includes switching means for stopping the upward movement of said platform when it is substantially disposed in alignment with said loading level, and interlocking means are provided for preventing movement of said platform above said position substantially aligned with said loading level until said locking bars are extended.

3. An arrangement as set forth in claim 2 wherein said interlocking means is connected to relay means for energizing further raising of said platform when said locking bars are extended, and said relay means including contactor means bypassing said switching means topermit raising of said platform above said position sub stantially disposed in alignment with said load level when said locking bars are extended.

4. An arrangement as set forth in claim 3 wherein locking bar circuit control means is connected to said actuating device for causing it to retract said locking bars, and said locking bar circuit control means being connected to actuate said relay means to inactivate said bypassing of said switching means when said locking bars are retracted.

5. An arrangement as set forth in claim 2 wherein said pressure bleeding means and said switching means include switches which are actuated in accordance with the vertical position of said platform, and said interlocking means comprise switches which are actuated in accordance with the extended and retracted conditions of said locking bars.

6. A vertically-movable platform arrangement comprising a hydraulic piston and cylinder assembly, a platform mounted upon said piston, said piston and cylinder assembly having a stroke sufficient to raise said platform through a series of levels including a loading level, a locking bar assembly connected to said platform including locking bars which are extendable to project laterally from said platform, an actuating device connected to said locking bar assembly for extending said bars laterally from said platform and for retracting them, supporting surface means disposed at the side of said platform adjacent said surface means whereby any forces imposed by a load moving across said platform are resisted by said supporting surface means and said hydraulic piston and cylinder assembly is relieved of any loads and pressures, said actuating device for said locking bar assembly comprising an auxiliary hydraulic piston and cylinder assembly which together with said hydraulic piston and cylinder assembly upon which said platform is mounted are supplied with pressure from a central hydraulic pressure generating system, said actuating device for said locking bar assembly includes an operating linkage employing a number of lever elements, said supporting surface means includes horizontal and vertical supporting surfaces which are engaged by said extended locking bars for resisting both vertical and horizontal shifting of said platform, and said operating linkage being arranged to align said lever elements in a substantially straight line when said locking bar assembly is extended for causing said linkage to rigidly resist any horizontal forces in their disposition between said vertical supporting surfaces.

7. A vertically-movable platform arrangement comprising a hydraulic piston and cylinder assembly, a platform mounted upon said piston, said piston and cylinder assembly having a stroke sufficicnt to raise said platform through a series of levels including a loading level, a docking bar assembly connected to said platform including locking bars which are extendable to project laterally from said platform, an actuating device connected to said locking bar assembly for extending said bars laterally from said platform and for retracting them, supporting surface means disposed at the side of said platform adjacent said loading level for engagement by said locking bars when they are extended, hydraulic pressure bleeding means connected to said piston and cylinder assembly for lowering said platform at a slow speed, control means for actuating said pressure bleeding means when said locking bars are extended above said supporting surface means and said platform is substantially aligned with said loading level to lower said platform upon said supporting surface means whereby any forces imposed by a load moving across said platform are resisted by said supporting surface means and said hydraulic piston and cylinder assembly is relieved of any loads and pressures, said piston and cylinder assembly being provided with pressurized hydraulic fluid from a pressure generating system, an electrically-operable lowering valve being connected in said system for controlling the return flow of fluid from said cylinder assembly to said system when said piston is being lowered, and said pressure bleeding means incorporating electrical control means which opens said electrically-operable valve when said platform is disposed above ground level.

8. An arrangement as set forth in claim 7 wherein low pressure switching means are connected to said system for closing said lowering valve and inactivating said pressure bleeding means when a minimum pressure exists in said cylinder.

9. An arrangement as set forth in claim 7 wherein additional switching means are provided for inactivating said pressure bleeding means when said platform is disposed above a predetermined distance from said loading level to permit said platform to raise and maintain a load above said predetermined distance.

10. A vertically-movable platform arrangement comprising a hydraulic piston and cylinder assembly, a platform mounted upon said piston, said piston and cylinder assembly having a stroke sufficient to raise said platform through a series of levels including a loading level, a locking bar assembly connected to said platform including locking bars which are extendable to project laterally from said platform, an actuating device connected to said locking bar assembly for extending said bars laterally from said platform and for retracting them, supporting surface means disposed at the side of said platform adjacent said loading level for engagement by said locking bars when they are extended, hydraulic pressure bleeding means connected to said piston and cylinder assembly for lowering said platform at a slow speed, control means for actuating said pressure bleeding means when said locking bars are extended above said supporting surface means and said platform is substantially aligned with said loading level to lower said platform upon said supporting surface means whereby any forces imposed by a load moving across said platform are resisted by said supporting surface means and said hydraulic piston and cylinder assembly is relieved of any loads and pressures, and the capacity of said pressure bleeding means being maintained slight enough to permit a normal supply of pressure fiuid to said cylinder to raise said piston even while a slight amount of pressure is bleeding therefrom.

11. A vertically-movable platform arrangement comprising a hydraulic piston and cylinder assembly, a platform mounted upon said piston, said piston and cylinder assembly having a stroke suflicient to raise said platform through a series of levels including a loading level, a locking bar assembly connected to said platform including locking bars which are extendable to project laterally from said platform, an actuating device connected to said locking bar assembly for extending said bars laterally from said platform and for retracting them, upporting surface means disposed at the side of said platform adjacent said loading level for engagement by said locking bars when they are extended, hydraulic pressure bleeding means connected to said piston and cylinder assembly for lowering said platform at a slow speed, control means for actuating said pressure bleeding means when said locking bars are extended above said supporting surface means and said platform is substantially aligned with said loading level to lower said platform upon said supporting surface means whereby any forces imposed by a load moving across said platform are resisted by said supporting surface means and said hydraulic piston and cylinder assembly is relieved of any loads and pressures, a hydraulic system incorporating control valves being connected to said piston and cylinder assembly for operating it, an auxiliary portion of said hydraulic system being connected to operate said actuating device for said locking bar assembly, said control means being connected to operate said control valves and including detecting elements associated with said platform arrangement, said control elements including detecting switches associated with said locking bar assembly for ascertaining when they are extended and retracted, and said control elements also including platform detecting switch means operated in accordance with the vertical position of said platform.

12. An arrangement as set forth in claim 11 wherein said platform detecting switch means includes a switch for preventing movement of said platform through a predetermined control position substantially aligned with said loading level until the condition of said locking bars is changed by extension when rising up through said position and retraction when lowering through said control position.

13. An arrangement as set forth in claim 12 wherein said platform detecting switch means prevents said platform from rising through said control position until said locking bars are extended, and said extended locking bars prevent said platform from dropping below said loading level.

14. An arrangement as set forth in claim 13 wherein said control means is arranged to lift said platform slightly above said loading level to said control position when said locking bars are being retracted to facilitate their movement.

15. A vertically-movable platform arrangement comprising a hydraulic piston and cylinder assembly, a platform mounted upon said piston, said piston and cylinder assembly having a stroke sufficient to raise said platform through a series of levels including a loading level, a locking bar assembly connected to said platform including locking bars which are extendable to project laterally from said platform, an actuating device connected to said locking bar assembly for extending said bars laterally from said platform and for retracting them, supporting surface means disposed at the side of said platform adjacent said loading level for engagement by said locking bars when they are extended, hydraulic pressure bleeding means connected to said piston and cylinder assembly for lowering said platform at a slow speed, control means for actuating said pressure bleeding means when said locking bars are extended above said supporting surface means and said platform is substantially aligned with said loading level to lower said platform upon said supporting surface means whereby any forces imposed by a load moving across said platform are resisted by said supporting surface means and said hydraulic piston and cylinder assembly is relieved of any loads and pressures, locking bar control circuit means being connected to said actuating device for energizing it to extend said locking bars, and said locking bar control circuit means being connected to said pressure bleeding means for actuating it as said locking bars are extended.

16. An arrangement as set forth in claim 15 wherein pressure-operated switch means is connected to said locking bar control circuit means for de-energizing it and preventing said locking bars from being retracted while the pressure in said locking bar control circuit is below a predetermined minimum.

References Cited in the file of this patent UNITED STATES PATENTS 488,838 Rowland Dec. 27, 1892 493,301 Rowland Mar. 14, 1893 2,377,483 Ellis June 5, 1945 2,655,114 Holdeman et al. Oct. 13, 1953 2,803,360 Midgley et al. Aug. 20, 1957 2,981,375 Borden Apr. 25, 1961 

1. A VERTICALLY-MOVABLE PLATFORM ARRANGEMENT COMPRISING A HYDRAULIC PISTON AND CYLINDER ASSEMBLY, A PLATFORM MOUNTED UPON SAID PISTON, SAID PISTON AND CYLNDER ASSEMBLY HAVING A STROKE SUFFICIENT TO RAISE SAID PLATFORM THROUGH A SERIES OF LEVELS INCLUDING A LOADING LEVEL, A LOCKING BAR ASSEMBLY CONNECTED TO SAID PLATFORM INCLUDING LOCKING BARS WHICH ARE EXTENDABLE TO PROJECT LATERALLY FROM SAID PLATFORM, AN ACTUATING DEVICE CONNECTED TO SAID LOCKING BAR ASSEMBLY FOR EXTENDING SAID BARS LATERALLY FROM SAID PLATFORM AND FOR RETRACTING THEM SUPPORTING SURFACE MEANS DISPOSED AT THE SIDE OF SAID PLATFORM ADJACENT SAID LOADING LEVEL FOR ENGAGEMENT BY SAID LOCKING BARS WHEN THEY ARE EXTENDED, HYDRAULIC PRESSURE BLEEDING MEANS CONNECTED TO SAID PISTON AND CYLINDER ASSEMBLY FOR LOWERING SAID PLATFORM AT A SLOW SPEED, CONTROL MEANS FOR ACTUATING SAID PRESSURE BLEEDING MEANS WHEN SAID LOCKING BARS ARE EXTENDING ABOVE SAID SUPPORTING SURFACE MEANS AND SAID PLATFORM IS SUBSTANTIALLY 